Mechanical destabilization of natural rubber latex adhesive formulations has been accomplished heretofore, with the use of hydrocarbon solvents, such as toluene. Solvents cause swelling which thin out the protective coating of surfactants after which the rubber molecules can easily make contact under finger-pressure and conglomerate, forming the adhesive bond. However, the use of solvents is not as desirable as it once had been considering the potential and real effects solvents have on the worker and the environment, both of which necessitate costly safeguards.
Mechanical destabilization of natural rubber has also been observed following the addition of an ethoxylated alcohol, T. D. Pendle, A. D. T. Gordon "The Mechanical Stability of Natural Rubber Latexes" Rubber Chemistry and Technology, Vol. 51, p. 986. The observation of the authors was that nonionic surfactants, added to latex mixes to confer chemical stability not attainable by the use of fatty acid soaps, had different effects upon the mechanical stability which effects were very dependent upon molecular structure of the surfactant. Octylcresol ethoxylates increased mechanical stability while tridecanol ethoxylates decreased mechanical stability.
It is also known in the art to reduce chemical stability of the latex primarily in order to form gelled film during latex dipping operations at elevated temperatures. Still other additives are employed to impart chemical stability to the latex in order to preserve, aid handling and storage and control coagulation. Chemical stability has been imparted to natural rubber latex by the addition of various surfactants and emulsifiers.
U.S. Pat. No. 3,006,872 discloses a method for heat-sensitizing natural rubber latex compositions by the addition of poly(ethylene oxide) which allows the latex to coagulate in or on hot mold surfaces thereby providing reduced chemical stability. Further disclosed is the use of alkylphenoxy polyoxyethylene ethanols as stabilizing agents to prevent coagulation of the latex upon the addition of acid thereby increasing mechanical stability.
U.S. Pat. No. 3,878,152 discloses a process for heat-sensitizing anionic and nonionic aqueous polymer dispersions, including both natural and synthetic rubber latexes, by the addition of alkoxylated amines of inverse solubility provided polyalkylene oxides of inverse solubility are also present and the pH of the dispersion is below 6. The patentees further state that the polymer dispersions stabilized in this manner are sufficiently stable to withstand shear forces during processing. Thus, chemical stability was reduced while mechanical stability was not.
Australian Pat. No. 146,350 discloses a process for softening natural rubber by heating an acid aqueous dispersion thereof containing a compound having an oxygen-oxygen bridge, in lieu of hydrogen peroxide, and a nonionic stabilizer, such as a polyethylene glycol ether, the latter being added prior to heating to render the dispersion stable under acid conditions and to prevent gelation.
These patents and others of which the foregoing are typical do not address the matter of reducing the mechanical stability of natural rubber latex merely by the use of nonionic surfactants. Controlling chemical stability is, however, sought and in most instances it is reduced. As is known to those skilled in the art, chemical stability can be reduced by the addition of various compounds but not without an accompanying increase in mechanical stability. To be useful in the quick-break adhesive art, it is desirable that the mechanical stability of natural rubber latex be reduced, allowing the latex to be made sensitive to pressure and/or shear and enabling it to be transformed from a colloidal liquid to a coherent film.